U.S. Pat. No. 4,401,507--issued August 30, 1983 describes one known method and apparatus for achieving spatially uniform, externally excited, non-thermal chemical reactions in the processing of semiconductor wafers. U.S. Pat. No. 4,610,748--issued September 9, 1986 discloses and claims a prior design for a semiconductor wafer holding fixture apparatus that could be used in the wafer processing method and apparatus described in U.S. Pat. No. 4,401,507. Wafer holding fixtures of the type diclosed in U.S. Pat. No. 4,610,748 function to hold semiconductor wafers (hereinafter referred to as workpieces) onto a flat, vertical, conductive electrode formed from a highly conductive grade of graphite. It is important that the wafer holding fixture hold the workpieces as upright (vertical) as possible and flat against the confronting face of its respective supporting electrode.
Holding fixtures for semiconductor wafers have had many different configurations and designs for holding the workpieces on the electrodes, all well below the center line of the workpiece. These include holding fixtures for supporting the workpieces at the bottom of the workpiece; fixtures for supporting the workpieces on the bottom and on one side; fixtures for supporting the workpieces at different angular positions below the center line of the workpiece with the holding fixture itself trying to force the workpiece against its supporting electrode, and the like. Other prior investigated methods and structures for holding workpieces properly positioned within a serial flow of reacting gases employ moving parts. Holding fixtures employing moving parts are thought to produce particulate contamination. It further appears that such methods and devices also produce a shadow effect on the workpiece and adversely affect the within-workpiece deposition uniformity. None of these prior art approaches solved the problem of properly supporting the workpiece flat against the confronting surface of its respective supporting electrode.
Workpieces being held onto a vertical electrode may tend to lean away from the electrode due to vibration or warping or thermal expansion. If this occurs, the workpiece will not be tight against the electrode and that adversely affects the deposition uniformity of the reactant gases on the workpiece. In these arrangements, the workpiece and electrode will usually be of different materials, thus causing a mismatch of thermal expansion and complicates the holding problem. It is known that the uniformity of the deposition of the reactant gases on the workpiece in these types of deposition systems, will be affected by the spacing and the electrical contact of the workpieces to their supporting electrodes. This is a random problem which prevents obtaining desired deposition coating thicknesses and uniformities.
It is believed that the different thermal expansion rates of the supporting electrodes and the workpieces involved are the major cause of the above briefly discussed holding problems. An example is a silicon wafer workpiece being held onto a graphite electrode. The graphite's thermal expansion is greater than the silicon wafer, so that when the graphite is heated, it will grow, causing the holding fixture for the silicon wafer to move apart. When the holding fixture moves apart, the silicon wafer will drop down further into the holding fixture. Thus, after completion of the deposition and the graphite electrode and fixture cool down, the fixture will pinch the completed silicon wafer preventing its easy removal and possibly resulting in chipping or ruining the wafer.
Past efforts to solve this thermal expansion problem resulted in designing the holding fixture so that the workpieces were held at points well below the center line of the workpiece. With such an arrangement, during the cool down of the electrodes, the workpiece can cam up onto the holding fixture for easy removal and not be pinched into position. However, where the workpiece is held well below its center line, problems due to leaning and displacement of the workpiece away from the confronting face of the supporting electrode as discussed above are accentuated. To overcome these problems, the present invention was devised.